1. Field of Application
In a general aspect, the present invention relates to a reactor for two-phase reactions, in particular for urea synthesis at high pressure and temperature of the type comprising:
a substantially cylindrical vertical external shell,
a plurality of superimposed perforated plates extending horizontally and in mutually spaced relationship in said shell, and
at least one opening for liquid flow defined in correspondence of each of said perforated plates.
The present invention also relates to in-situ modernization of reactors for two-phase reactions, in particular for urea synthesis at high pressure and temperature.
In the description given below and in the following claims, the term: xe2x80x9cin-situxe2x80x9d modernization, is understood to mean the on-site modification of a pre-existing reactor in order to improve its performance and obtain e.g. greater production capacity and/or greater conversion yield and/or reduction in energy consumption.
In the terminology of the field this type of modernization is also termed xe2x80x9cretrofittingxe2x80x9d or xe2x80x9crevampingxe2x80x9d.
In the field of two-phase reactions at high pressure and temperature, e.g. for urea synthesis or hydrolysis, the requirement for increasing the conversion yield of synthesis reactors to improve their production capacity and reduce energy plant consumption in which said reactors operate is increasingly felt.
2. Prior Art
In order to satisfy said requirement synthesis reactors comprising a vertical tubular shell in which is supported a plurality of superimposed horizontal perforated plates in mutually spaced relationship have been becoming increasingly used.
The reaction product, e.g. urea, is obtained by placing in intimate contact a liquid phase and a gaseous phase comprising ammonia and carbon dioxide (CO2) flowing in the shell from below upward.
The perforated plates have the function of mixing together said phases to facilitate their intimate contact and hence exchange of mass and heat indispensable for conversion of the reagents, ammonia and CO2, into urea.
Synthesis reactors for two-phase reactions in accordance with the prior art are mainly of two types depending on the perforated plates used.
A first type of reactor as shown in FIG. 1 comprises a plurality of superimposed perforated plates extending horizontally over the entire cross section of the reactor and in which is defined a plurality of holes for the passing of a two-phase gas and liquid flow.
Since the liquid and gaseous phases pass through the same holes, there is alternating passing of gas and liquid with a pulsing flow which prevents intimate gas and liquid mixing. As a result there are low mass and heat transfer coefficients and hence low conversion yield.
In another case as shown in FIG. 3 the synthesis reactor comprises a plurality of superimposed horizontal perforated plates in mutually spaced relationship. Between the perimetric edge of each of these plates and the internal wall of the reactor is defined an annular aperture.
Even in this case however it is not possible to obtain the desired intimate mixing between the liquid phase and the gaseous phase because the liquid flows preferentially along said peripheral apertures while the gas tends to coalesce in the central part of the reactor.
Since they do not ensure effective intimate contact between the reagents the reactors in accordance with the prior art are not able to permit an optimal exchange of material and heat, which is the basic condition for achieving optimal conversion yield. Said reactors operate therefore far below their potential production capacity with resulting high energy consumption of the plant, e.g. for urea production, in which said reactors operate.
The technical problem underlying the present invention is to provide synthesis reactors capable of operating with high conversion yields so as to obtain greater production capacity and much lower energy consumption than those of the plants in accordance with the prior art mentioned above.
The above technical problem is solved in accordance with the present invention by a reactor of the above type, and characterized in that the openings for liquid flow defined in correspondence of at least two adjacent perforated plates are mutually offset.
In the description given below and in the following claims, the term: mutually offset openings, is understood to mean a plurality of openings essentially designed for the passing of the liquid phase, whose projection is not superimposed on the openings defined in correspondence of the adjacent perforated plates.
It has been found that thanks to the reactor in accordance with the present invention the liquid phase flows in the latter along an substantially zigzag preferential flow path which crosses the essentially vertical flow path of the gaseous phase.
In this manner it is possible to obtain continuous mixing of said phases along the entire flow path defined between adjacent perforated plates. There is thus advantageously increased the intimate contact between reagents with resulting increase of the mass and heat transfer coefficient between liquid and gas.
Advantageously the reactor in accordance with the present invention allows achievement of a high conversion yield optimizing the production capacity thereof and minimizing the energy consumption of the plant in which the reactor operates.
In a particularly advantageous and preferred embodiment of the present reactor at least one of the adjacent perforated plates is divided in a plurality of perforated sectors and unperforated sectors side by side.
Advantageously the perforated and unperforated sectors extend on said at least one perforated plate between an edge adjacent to the opening for liquid flow and an opposing edge adjacent to an internal wall of the shell.
Thanks to this particular configuration of the perforated plates it is possible to further increase the mixing between the gaseous phase and the liquid phase and hence the reactor yield.
Indeed the presence in the plates of the unperforated sectors facilitates penetration of the liquid phase flowing from one plate to the other along an essentially zigzag flow path into the gaseous phase which traverses the reactor along an essentially vertical flow path.
Practically, the liquid phase when it runs over a perforated plate is divided in a plurality of flows which cross the flows of the gaseous phase coming out from the perforated sectors of the plate. The gaseous flows in turn cause aspiration of the liquid flows running beside them.
This plurality of alternating liquid and gaseous flows in direct contact with each other has as a result an increase in the exchange surface between the phases during the passing of the latter from one plate to the next and thus causes an increase in the intimate contact between the reagents which facilitates transfer of material and heat.
The result obtained by the division of at least one perforated plate in perforated and unperforated sectors is thus faster and more intimate mixing of the liquid phase and the gaseous phase.
It was also found that optimal mixing is obtained when the perforated and unperforated sectors are substantially rectilinear, parallel and preferably when they have equal width.
Advantageously the openings for liquid flow in the reactor in accordance with the present invention are made up of diametrically opposed parts of the adjacent plates so as to maximize the liquid phase flow path between adjacent perforated plates to increase the intimate contact between the reagents.
In accordance with another aspect of the presente invention there is also made available an in-situ modernization method for a reactor for two-phase reactions, in particular for urea synthesis at high pressure and temperature of the type wherein a co-current flow of a gaseous phase and a liquid phase takes place.
In a first embodiment the method of the present invention calls for in-situ modernization of a reactor for two-phase reactions, in particular for urea synthesis at high pressure and temperature of the type wherein a co-current flow of a gaseous phase and a liquid phase takes place, comprising a vertical tubular shell in which is supported a plurality of superimposed perforated plates in mutually spaced relationship, with said plates extending horizontally in said shell for the entire cross section thereof. In this case, the method is characterized in that it comprises the step of forming in at least two adjacent perforated plates respective mutually offset openings for liquid flow.
In a second embodiment the method in accordance with the present invention calls for in-situ modernization of a reactor for two-phase reactions, in particular for urea synthesis at high pressure and temperature of the type wherein a co-current flow of a gaseous phase and a liquid phase takes place, comprising a vertical tubular shell in which is supported a plurality of superimposed horizontal perforated plates in mutually spaced relationship, with at least one aperture defined between a perimetric edge of each of said plates and an internal wall of said shell. In this case the, method is characterized in that it comprises the step of partially obstructing the apertures defined in correspondence of at least two adjacent plates by means of baffles, with said baffles defining respective mutually offset openings for liquid flow.
In a third embodiment the method in accordance with the present invention calls for in-situ modernization of a reactor for two-phase reactions, in particular for urea synthesis at high pressure and temperature of the type wherein a co-current flow of a gaseous phase and a liquid phase takes place, comprising a vertical tubular shell in which is supported a plurality of superimposed horizontal perforated plates in mutually spaced relationship, with at least one aperture defined between a perimetric edge of each of said plates and an internal wall of said shell. In this case the, method is characterized in that it comprises the steps of obstructing the apertures defined in correspondence of at least two adjacent plates by means of baffles, and forming in said adjacent plates respective mutually offset openings for liquid flow.
In a preferred embodiment, the method of the present invention comprises additionally the step of providing in at least one of the adjacent perforated plates a plurality of perforated and unperforated sectors side by side.
Preferably the perforated and unperforated sectors side by side are provided by obstructing in preset zones the holes present in said at least one of said adjacent perforated plates.
As an alternative, the method of the present invention comprises additionally the step of providing in the shell at least one perforated plate comprising a plurality of perforated and unperforated sectors side by side.
The characteristics and advantages of the present invention are set forth in the description of an embodiment thereof given below by way of non-limiting example with reference to the annexed-drawings.
Said description relates in particular to urea synthesis at high pressure and temperature. It is however clear that the following description can also be applied to other types of two-phase reactions such as for example urea hydrolysis reaction.